1. Field of the Invention
The present invention relates to new water-borne, two-component polyurethane coating compositions based on selected polyhydroxyl compounds dispersed in aqueous medium and polyisocyanates emulsified in these dispersions, to a process for the production of these coating compositions by emulsification of the polyisocyanate component in the dispersion and to the use of the coating compositions for the production of coatings on water-resistant substrates.
2. Description of the Prior Art
Ecological factors play an important part in surface technology. A particularly urgent problem is reducing the quantities of organic solvents used in coating compositions.
Until recently it had not been possible to dispense with organic solvents in chemically crosslinking polyurethane coating compositions, which have acquired considerable significance in the coating field due to their outstanding properties. The use of water instead of organic solvents in these two-component polyurethane coating compositions wherein the polyisocyanate component contained free isocyanate groups did not appear possible because it is known that isocyanate groups react not only with alcoholic hydroxyl groups, but also with water. Because the concentration of active hydrogen atoms emanating from water in these systems is substantially greater than the concentration of hydroxyl groups from the organic NCO-reactive component, it had to be assumed that in the ternary polyisocyanate/organic polyhydroxyl compound/water system a reaction would take place between the isocyanate groups and water to form urea and carbon dioxide. Instead of the desired crosslinking reaction between the polyisocyanate and the organic polyhydroxyl compounds, it was expected that the isocyanate groups would react with water resulting in foam formation through the generation of carbon dioxide.
It is already known from DE-OS 2,708,442 and from DE-OS 3,529,249 that organic polyisocyanates may be added to aqueous polymer dispersions to improve their property spectrum. However, the polymers disclosed therein are not the organic polyhydroxyl compounds typically used as reactants for polyisocyanates. In addition, the effect of adding polyisocyanates to aqueous polymer dispersions described in these prior publications is presumably attributable to coating the dispersed polymer with the urea formed from the reaction between the polyisocyanate and water.
It was only from European Patent Application EP-A-0,358,979 that it became known that selected polyhydroxyl compounds based on vinyl polymers could be used as reactants for organic polyisocyanates containing free isocyanate groups for the production of water-borne, two-component polyurethane systems by emulsification of the polyisocyanates containing free isocyanate groups in the aqueous polymer solution or dispersion.
The polyhydroxyl compounds described in EP-A-0,358,979 are preferably prepared by free radical polymerization and have an acid value (based on the non-neutralized sulfonic acid and/or carboxyl groups) of 0 to 150, preferably 0 to 100 mg KOH/g solid resin; and a content of sulfonate and/or carboxylate groups of 5 to 417, preferably 24 to 278 milliequivalents per 100 g solids.
It has now been surprisingly found that even when vinyl polymer polyols containing less than 5 milliequivalents of sulfonate and/or carboxylate groups per 100 g of solid resin are used, it is possible to obtain two-component polyurethane coating compositions having very good properties, including long pot lives and high solvent resistance.
The present invention relates to water-borne coating compositions wherein the binder contains a mixture of
a) a polyol component dispersed in water or a water/solvent mixture which contains at least one polymer prepared from olefinically unsaturated monomers and having a molecular weight (Mn) of at least 500, at least two alcoholic hydroxyl groups per molecule, a hydroxyl value of 15 to 250 mg KOH/g, an acid value of 0 to 7 mg KOH/g and a total content of sulfonate and carboxylate groups of 0 to 4.5 milliquivalents per 100 g of solid resin and
b) a polyisocyanate component which is emulsified in the dispersion of polyol component a), has a viscosity at 23xc2x0 C. of 50 to 10,000 mPa.s and an average NCO functionality of 1.8 to 4.2, and contains 12.0 to 21.5% by weight of (cyclo)aliphatically bound isocyanate groups and, optionally, 2 to 20% by weight of ethylene oxide units present within polyether chains, the polyether chains containing a statistical average of 5 to 70 ethylene oxide units,
wherein the components are present in quantities corresponding to an equivalent ratio of isocyanate groups of component b) to alcoholic hydroxyl groups of component a) of 0.2:1 to 5:1.
The present invention also relates to a process for the production of these coating compositions by emulsifying the polyisocyanate component in an aqueous or aqueous/organic dispersion of the polyol component in amounts corresponding to an equivalent ratio of isocyanate groups to alcoholic hydroxyl groups of 0.2:1 to 5:1 and, prior to the addition of the polyisocyanate, incorporating any auxiliaries and additives into the polyol component.
Finally, the present invention relates to coated water-resistant substrates prepared from these coating compositions.
The polyol component is selected from polymer resins prepared from olefinically unsaturated monomers which have a molecular weight (Mn, as determined by gel permeation chromatography) of at least 500, preferably 500 to 100,000 and more preferably 1000 to 50,000; an average of at least two alcoholic hydroxyl groups per molecule; a hydroxyl value of 15 to 250, preferably 30 to 150, mg KOH/g solids; an acid value (based on unneutralized sulfonic acid and carboxyl groups) of 0 to 7, preferably 0, mg KOH/g solid resin; and a total content of sulfonate and carboxylate groups of 0 to 4.5, preferably 0, milliequivalents per 100 g of solid resin.
The dispersions a) generally contain 10 to 50% by weight, preferably 20 to 40% by weight, of the polyol component as the dispersed phase. The continuous aqueous phase may contain up to 20% by weight, based on the weight of the continuous phase, of the organic solvents to be described hereinafter. Accordingly, the continuous phase of the dispersions is primarily water. The dispersions generally have a viscosity at 23xc2x0 C. of 10 to 10,000, preferably 100 to 10,000 mPa.s; and a pH of 5 to 10, preferably 6 to 9.
The dispersibility of the polymer resins is based on the presence of the emulsifiers to be described hereinafter. In addition, in a less preferred embodiment, component a) may contain up to 10% by weight, based on the weight of the polyol component, of water-soluble polyhydric alcohols having a molecular weight of 62 to 499, preferably 62 to 200, which are different from the solvents to be described hereinafter. Examples of these alcohols include ethylene glycol, propylene glycol, glycerol, trimethylol propane and the low molecular weight, water-soluble alkoxylation products of these polyhydric alcohols.
The polyol component is produced by known radical polymerization methods, for example, solution polymerization, emulsion polymerization and suspension polymerization. They are preferably produced by radical emulsion polymerization in aqueous medium.
Continuous or discontinuous polymerization processes may be applied. Of the discontinuous processes, the batch process and the inflow process may be used; the inflow process is preferred. In the inflow process, water is initially introduced either alone or with a portion of the anionic emulsifier, an optional nonionic emulsifier and a portion of the monomer mixture, and heated to the polymerization temperature. After introduction of a portion of the monomers the polymerization reaction is radically initiated. The remainder of the monomer mixture is added together with the remainder of the initiator mixture and the emulsifier over a period of 1 to 10 hours, preferably 3 to 6 hours. If necessary, more activator may then be added to enable the polymerization to be continued to a conversion of at least 99%.
The emulsifiers used are anionic and/or nonionic emulsifiers. Preferred anionic emulsifiers are those containing carboxylate groups, sulfate groups, sulfonate groups, phosphate groups or phosphonate groups. Emulsifiers containing sulfate, sulfonate, phosphate or phosphonate groups are preferred. The emulsifiers may have low or high molecular weights. High molecular weight emulsifiers are described, for example, in DE-OS 3,806,066 and in DE-AS 1,953,349.
Preferred anionic emulsifiers are synthesized from long-chain alcohols or substituted phenols and a monohydroxy polyether containing ethylene oxide units and having a degree of polymerization of 2 to 100, wherein the polyether chain also contains a sulfuric acid or phosphoric acid group attached in the form of an ester unit. Preferred neutralizing agents for the unesterified acid groups are ammonia or amines. The emulsifiers may be added to the emulsion individually or in admixture.
Suitable nonionic emulsifiers, which may be used in combination with the anionic emulsifiers, are reaction products of aliphatic, araliphatic, cycloaliphatic or aromatic carboxylic acids, alcohols, phenol derivatives or amines with epoxides, such as ethylene oxide. Examples are reaction products of ethylene oxide with carboxylic acids of castor oil or abietic acid; relatively long-chain alcohols such as oleyl alcohol, lauryl alcohol and stearyl alcohol; phenol derivatives such as substituted benzyl, phenyl and nonylphenols; and relatively long-chain amines such as dodecyl amine and stearyl amine. The reaction products with ethylene oxide are oligoethers and polyethers having degrees of polymerization of 2 to 100, preferably 5 to 50.
The emulsifiers are added in quantities of 0.1 to 10% by weight, based on the mixture of monomers. Suitable co-solvents, which may optionally be used, are both water-soluble and water-insoluble solvents, e.g., aromatic hydrocarbons such as benzene, toluene, xylene and chlorobenzene; esters such as ethyl acetate, butyl acetate, methyl glycol acetate, ethyl glycol acetate and methoxypropyl acetate; ethers such as butyl glycol, tetrahydrofuran, dioxane, ethyl glycol ether and ethers of diglycol; ketones such as acetone, methy ethyl ketone and methyl isobutyl ketone; trichloromonofluoroethane; and cyclic amides such as N-methyl pyrrolidone and N-methyl caprolactam.
The radically initiated polymerization may be initiated by water-soluble or water-insoluble initiators or initiator mixtures having radical dissociation half lives at temperatures of 10 to 100xc2x0 C. of 0.01 to 400 minutes.
In general, the polymerization reaction takes place in aqueous emulsion at a temperature of 10 to 100xc2x0 C., preferably 30 to 90xc2x0 C., under a pressure of 1000 to 20,000 mbar. The actual polymerization temperature is determined by initiator used. The initiators are used in quantities of 0.05 to 6% by weight, based on the total quantity of monomers.
Suitable monomers include hydroxy-functional monomers, acidic monomers and monomers which do not contain either hydroxyl or acidic groups. These monomers are disclosed in EP-A-0,358,979 (U.S. Pat. No. 5,075,370, which is herein incorporated by reference). Acidic monomers, i.e., monomers containing carboxyl and/or sulfonic acid groups, are only used in the small quantities previously set forth and preferably are not used at all.
Suitable initiators include water-soluble and water-insoluble azo compounds such as azoisobutyrodinitrile and 4,4xe2x80x2-azo-bis-(4-cyanopentanoic acid); inorganic and organic peroxides such as dibenzoyl peroxide, t-butyl perpivalate, t-butyl per-2-ethylhexanoate, t-butyl perbenzoate, t-butyl hydroperoxide, di-t-butyl peroxide, cumene hydroperoxide, dicyclohexyl and dibenzyl peroxydicarbonate, and the sodium, potassium or ammonium salts of peroxodisulfuric acid and hydrogen peroxide. The peroxodisulfates and hydrogen peroxides are often used in combination with reducing agents such as the sodium salt of formamidine sulfinic acid (Rongalit C), ascorbic acid or polyalkylene polyamines. A reduction in the polymerization temperature is generally obtained in this way.
The molecular weight of the polymers may be regulated by the use of typical regulators such as n-dodecyl mercaptan, t-dodecyl mercaptan, diisopropyl xanthogene disulfide, di(methylenetrimethylolpropane) xanthogene disulfide and thioglycol. They are used in quantities of at most 3% by weight, based on the monomer mixture.
After the polymerization reaction is complete, neutralizing agents are optionally added to the polymers present in aqueous dispersion to provide a degree of neutralization of 30 to 100%, preferably 50 to 100%.
Inorganic bases, ammonia or amines may be used as neutralizing agents. Suitable inorganic bases include sodium hydroxide and potassium hydroxide. In addition to ammonia, suitable amines include trimethyl amine, triethyl amine, dimethyl ethanolamine, methyl diethanolamine, triethanolamine, etc. The neutralizing agents may be used both in less than and greater than equivalent quantities. After neutralization the polymers have the previously mentioned acid numbers and contents of sulfonate and carboxylate groups, preferably carboxylate groups.
In cases where the acidic groups optionally present are completely neutralized, the acid value is zero while the content of sulfonate and/or carboxylate groups corresponds to the original content of sulfonic acid groups or carboxyl groups. If the acidic groups are partially neutralized, the content of sulfonate and/or carboxylate groups corresponds to the quantity of neutralizing agent used. The aqueous dispersions obtained have the above-mentioned concentrations and viscosities. Any co-solvents added may remain in the aqueous dispersion in the quantities mentioned above or may even be removed by distillation on completion of the polymerization reaction.
Polyisocyanates suitable as crosslinking component b) include polyisocyanate mixtures having
a) an average NCO functionality of 1.8 to 4.2,
b) a content of (cyclo)aliphatically bound isocianate groups (expressed as NCO, molecular weight=42) of 12.0 to 21.5% by weight and, preferably,
c) a content of ethylene oxide units present within polyether chains (expressed as C2H4O, molecular weight=44) of 2 to 20% by weight, the polyether chains containing on an average of 5 to 70 ethylene oxide units.
These preferred polyisocyanate mixtures are prepared in known manner by the reaction of a polyisocyanate component A) having an (average) NCO functionality of 2.1 to 4.4, preferably 2.3 to 4.3, and containing one or more polyisocyanates having only (cyclo)aliphatically bound isocyanate groups with a monofunctional or polyfunctional polyalkylene oxide polyether alcohol B) containing an average of 5 to 70 ethylene oxide units, an NCO:OH equivalent ratio of least 2:1 and generally from 4:1 to approx. 100:1 being maintained during the reaction. In addition, the type of and quantitative ratios between the starting components mentioned are selected so that the reaction products obtained comply with the conditions mentioned above under a) to c).
The polyisocyanates or polyisocyanate mixtures A) are polyisocyanate derivatives containing uretdione, isocyanurate, urethane, allophanate, biuret and/or oxadiazine trione groups prepared from monomeric (cyclo)aliphatic diisocyanates. Processes for the preparation of the polyisocyanate derivatives are described in DE-OSS 1,670,666, 3,700,209 and 3,900,053 or in EP-A 336,205 and 339,396.
Suitable (cyclo)aliphatic diisocyanates for the production of the polyisocyanates derivatives are those having a molecular weight of 140 to 400 such as 1,4-diisocyanatobutane, 1,6-diiso-cyanatohexane, 1,5-diisocyanato-2,2-dimethyl pentane, 2,2,4-and 2,4,4-trimethyl-1,6-diisocyanatohexane, 1,10-diisocyanato-decane, 1,3- and 1,4-diisocyanatocyclohexane, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane (isophorone diisocyanate), 4,4xe2x80x2-diisocyanatodicyclohexyl methane and mixtures of these diisocyanates.
Preferred starting components for the preparation of the hydrophilically-modified polyisocyanates are polyisocyanate mixtures having an NCO content of 1 g to 24% by weight and containing the isocyanurate trimer 1,6-diisocyanatohexane and the isocyanurate dimer of 1,6-diisocyanatohexane. Another preferred starting component is a polyisocyanate having an NCO content of 1 g to 24% by weight which contains the isocyanurate trimer 1,6-diisocyanatohexane but does not contain the uretdione dimer. This polyisocyanate preferably has an average NCO functionality of 3.2 to 4.2.
Monohydric or polyhydric polyalkylene oxide polyether alcohols B) contain an average of 5 to 70, preferably 6 to 60, ethylene oxide units per molecule and may be obtained in known manner by the alkoxylation of suitable starter molecules.
Monohydric or polyhydric alcohols having a molecular weight of 32 to 150, for example those disclosed in EP-A 206,059 (U.S. Pat. No. 4,663,377, herein incorporated by reference), may be used as starter molecules for the production of the polyether alcohols B). Monohydric aliphatic alcohols containing 1 to 4 carbon atoms are preferably used as starter molecules. Methanol is particularly preferred.
Alkylene oxides suitable for the alkoxylation reaction are ethylene oxide and optionally other oxides such as propylene oxide. When mixtures of oxides are used they may be added sequentially and/or in admixture during the alkoxylation reaction. Accordingly, the polyalkylene oxide polyether alcohols B) are either pure polyethylene oxide polyethers or mixed polyalkylene oxide polyethers which contain at least one polyether chain containing at least 5, preferably 5 to 70, more preferably 6 to 60 and most preferably 7 to 20, ethylene oxide units; and in which at least 60 mole %, preferably at least 70 mole %, of the alkylene oxide units are ethylene oxide units.
Preferred polyether alcohols B) for the production of the water-emulsifiable polyisocyanate mixtures are monofunctional polyalkylene oxide polyethers started with an aliphatic C1-4 alcohol and containing an average of 6 to 60 ethylene oxide units. Particularly preferred polyether alcohols B) are pure polyethylene glycol monomethyl ether alcohols containing an average of 7 to 20 ethylene oxide units.
Instead of the preferred nonionically/hydrophilically modified polyisocyanates described above, unmodified polyisocyanates A) may also be used as component b) or a portion of component b), provided that the polyisocyanates are used in combination with suitable emulsifiers. Examples include the emulsifiers described, for example, in EP-A 0 013 112 for providing hydrophilicity to aromatic polyisocyanates, or the emulsifiers previously disclosed herein to be used with polyol component a). The emulsifiers are present in sufficiently large quantities to ensure the dispersibility of the hydrophobic polyisocyanates in the system.
It is also possible in principle, although less preferred, to also use anionically/hydrophilically modified polyisocyanates b). These polyisocyanates may be prepared by reacting polyisocyanate derivatives A) with less than equivalent quantities of dimethylol propionic acid and subsequently neutralizing the carboxyl group with tertiary amines.
Finally, water-emulsifiable aromatic polyisocyanate mixtures such as those described in GB-PS 1,444,933, in DE-OS 2,921,681 and in EP-A-61,628 are also suitable, but less preferred, as crosslinking component b).
To produce the ready-to-use coating composition, polyisocyanate component b) is emulsified in the aqueous dispersion of polyol component a). Mixing may also be carried out simply by stirring at room temperature.
The quantity in which the polyisocyanate component is used is selected to provide an NCO:OH equivalent ratio, based on the isocyanate groups of component b) and the alcoholic hydroxyl groups of component a), including the hydroxyl groups of any water-soluble, low molecular weight polyhydroxyl compounds used, of 0.2:1 to 5:1, preferably 0.5:1 to 2:1. Before the addition of polyisocyanate component b), the known auxiliaries and additives used in coatings technology may be incorporated in polyol component a). The auxiliaries and additives include foam inhibitors, flow control agents, thickeners, pigments, dispersion aids for the dispersion of pigments, etc.
The coating compositions according to the invention obtained as described above are suitable for virtually any applications where solvent-containing, solvent-free or other water-borne paint and coating systems having superior property profiles are presently used. Examples include the coating of any mineral building materials, such as lime- and/or cement-bonded plasters, gypsum-containing surfaces, fiber cement building materials and concrete; painting and sealing of wood and wood materials, such as chipboard, fiber board and paper; painting and coating of metallic surfaces; coating and painting of asphalt- and bitumen-containing pavements; paint and sealing of various plastic surfaces; and coating of leather and textiles. They are also suitable for surface-to-surface bonding of various materials and may be used for bonding the same or different materials to one another.
After it has been applied to the particular substrate, the two-component system may be cured or crosslinked at a temperature of 0 to 300xc2x0 C., preferably at a temperature of room temperature to 200xc2x0 C.